AVS 46th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS2-TuM

Paper SS2-TuM3
Structurally Ordered Magnesium Vanadate Model Catalysts for Oxidative Dehydrogenation

Tuesday, October 26, 1999, 9:00 am, Room 607

Session: Model Catalysts
Presenter: A.G. Sault, Sandia National Laboratories
Authors: A.G. Sault, Sandia National Laboratories
J.A. Ruffner, Sandia National Laboratories
Correspondent: Click to Email
A fundamental understanding of the nature of the active sites in mixed metal oxide oxidation catalysts has proven elusive. The complex nature of these materials makes fundamental studies of active sites difficult. The use of single crystal surfaces to overcome this complexity is limited by the insulating nature of oxides, which restricts the use of many surface analytical probes. For simple oxides, some researchers have used ultrathin epitaxial oxide films to overcome these problems. Such films allow STM studies that provide unprecedented atomic detail regarding metal oxide surfaces. We are employing similar methods to study mixed metal oxide surfaces. Using RF sputter deposition, we have grown thin (<85 Å) films of Mg@sub 3@(VO@sub 4@)@sub 2@, a known selective oxidative catalyst. Bulk and surface analysis of films grown on Si wafers show the desired stoichiometry. By depositing Mg@sub 3@(VO@sub 4@)@sub 2@ on a 500 Å Au layer grown on an oxidized Si wafer, films strongly orientated in the (042) plane of Mg@sub 3@(VO@sub 4@)@sub 2@ can be grown. This plane consists of close packed oxygen layers, with Mg and V ions in octahedral and tetrahedral sites, respectively. Heating the films above 623 K in vacuum or 100 Torr propane results in partial reduction of V(V) to V(IV) and segregation of V to the surface. Treatment in oxygen reverses these changes. This reversible redox chemistry is consistent with the known mechanism for catalytic oxidation, which involves participation of lattice oxygen. We will report STM images of these surfaces, and detail the effects of heat treatments in reactive environments (e.g., oxygen/propane mixtures) on the structure and composition of these films. The formation of defects such as oxygen vacancies will be of special interest as lattice oxygen plays an important role in catalysis over mixed metal oxides. Through these studies we hope to provide the first detailed atomic scale view of active sites in this important class of catalysts.